do a full virus scan , go to the hard drive settings and run the disc clean , how long since you defraged it ? do thgat as well, op[en up the back up system and remove backups older than say a week or month, load and run adaware that will sort out some nasties , I have downloaded a program called crap cleaner , that sorts out all the rubbish , and deletes it ,

Being shot out of a cannon will always be better than being squeezed out of a tube. That is why God made fast motorcycles,
Hunter S Thompson

if you use windows 8 or 8.1 (or even 10) then its really easy to diagnose system issues using the task manager (ctrl shift esc) make sure your HDD (hard drive C:\) isnt being overwhelmed (high usage) if it is then post a screenshot should be able to diagnose further

All good advice above, CCleaner is good at finding and removing unnecessary files but it also has a Registry cleaner facility which is really useful for resolving any issues with the registry which might be causing problems, I'd recommend running both the disk cleaner and the registry cleaner.

Make sure your antivirus is up to date and run a full scan, and another vote for Malwarebytes.

You don't say how old your laptop is or what operating system it's running. Is it getting hot, as sometimes the CPU cooler can get clogged with dust meaning it doesn't work very well which then causes the CPU to overheat, the CPU will then start to run at slower speeds to try to cool down which will noticeably affect performance.

Have a look in the performance section of task manager when the laptop is running slow and check whether the CPU, memory or disk is running at 100% as this will show you whether there is a bottleneck. Also if you are running programs from your external hard drive rather than just using it to store data this could affect performance as your USB connections data transfer rate will possibly not be fast enough.